This invention is directed to a dust collection system as used in cabinetmaking, carpentry and similar trades, or in any industrial setting in which process dust is generated and needs to be controlled. In some cases, flammable process dust is generated and can create risk of fire or explosion. In the dust collection system, the process dust is directed from dust producing equipment to a dust storage drum and the airflow is separated from the dust is returned to the ambient. In favorable embodiments, the dust collection system may involve a cyclonic dust separator, followed by a HEPA filter or similar final air filter.
The invention is more specifically directed to a dust separator in which the motor that drives the blower or impeller automatically adjusts rotation speed to match the air flow demand and matches the impedance to air flow imposed by the dust producing tools and ducts or hoses to which it is attached, so that the dust separation is optimized. This is most favorably achieved with a variable frequency motor drive and a motor load current feedback control.
The invention is also directed to dust collector systems with a feature for suppressing or arresting flame that may come about by inadvertent ignition of the dust in the process air stream, so as to prevent deflagration and keep flame from igniting dust in the ambient air.
The invention is furthermore directed to a Vee design cyclonic dust collector in which the cyclonic chamber involves a single cone with the intake of air and process dust entering through a penetration in the conic wall, with the process dust separating and descending, leaving the lower nose end of the cone, and with the air leaving the cone through a vortex tube that penetrates a divider plate at the top of the conic body. A fan chamber then is positioned atop the divider plate and conic body, generally rising from the top of the conic body (or slightly outside the radius of the top of the conic body). The fan in the chamber impels the airflow through a ductway to a final filter, and from there the filtered air returns to the ambient. The Vee design of the cyclonic separator has advantages in manufacturing costs and in separation efficiency.
Factories, home workshops, and other operations that utilize cutting tools (e.g., for wood, metal, or plastic) or some other type of debris-producing machinery generally need to incorporate a dust collection or air cleaning system to ensure safe working conditions, quality machining, good air quality, and to satisfy certain government environmental and safety regulations. Typically, debris-producing machines have a vacuum hose attached to a dust port that is located in close proximity to the cutting or grinding operation that is used to entrain and extract the debris, i.e., wood chips, filings and fines produced by the machine. The production dust is carried away through a conduit or flexible tubing and eventually into to the dust collector.
These operations typically have a range of processing tools that are used selectively as needed and are not all operated simultaneously. For example, there may be twelve processing machines where at times just a few, or perhaps only one machine is operated and at other times most or all of the machines may be operated. These machines can require significantly different air volumes and pressures to effectively extract material and entrain debris into the system. They also have a plurality of different dust ports sizes (diameters) and highly variable dust collection requirements. For example, a 20-inch wood planer may have a six-inch-diameter dust collection port and may require 800 CFM at 8 inches static pressure (measured in terms of water column). On the other hand, a small CNC router may require 250 CFM at 23 inches of static pressure measured in water column, and may have a 2½″ dust port. Currently, two separate and independent dust collection systems of different design types would be required to meet these specifications. Furthermore, it is not uncommon for manufacturers to have poorly designed or undersized dust ports integrated into the machine, requiring much higher pressures and velocities than is recommended to meet the best-practice standard.
If the amperage load on the blower motor decreases, the impeller speed would need to increase to restore a full-load motor condition. Alternatively, the inverse is true. If dust conduit lengths are increased or different ports sizes are opened and closed, the motor load changes as well. However, at present it has not been possible to automatically adjust motor speed to accommodate changing conditions.
It is also quite important that velocity of the air in the ducts should not fall below minimum conveying velocities. The need for this is twofold:                a) When material is conveyed in a pipe or duct air velocity must be maintained at or above a minimum speed (contingent on the density of the conveyed material) or material will drop out of the airstream, clogging the duct. Larger collectors that collect from a plurality of variable duct and ports sizes are unable to maintain sufficient conveying velocities and lack flexibility of volume and pressure capability to alternate between high volume and low pressure to high pressure and lower volume requirements to overcome higher friction losses in smaller diameter ducts, dust ports and hoses        b) Also, air-to-dust ratios (concentration level of air to dust) of finely divided material is an important consideration for combustible dusts in maintaining a mixture concentration that is below the Lower Flammability Limit (LFL) to prevent a hazard of fire or deflagration. Lower Flammability Limit (LFL) is the point of concentration where the dust component of the dust-air mixture is just sufficient to support combustion if a spark or ignition source enters the duct system. For Example, the LFL for finely divided wood dust is 40 grams per cubic meter, or 1.133 g/cu ft. A concentration below 40 grams per cubic meter will not deflagrate or combust.        
Problems exist in the current state of the art where a dust collection radial fan blower, of the type used in material conveying systems, delivers a pressure-to-volume characteristic that is a well-defined single function or curve on a X-axis/Y-axis chart, considered where impeller speeds are constant. These radial fan blowers are designed for a narrow band of airflows and pressure ranges. The drawbacks of this design are numerous, as the air flow demand can easily go outside the design airflow band of the blower. This occurs when tools are added to, or taken off the system.
Direct-drive radial fans (which are often used in dust collection equipment) move less air as the inlets and outlets are restricted, resulting in less work performed and less amperage draw on the motor. For example, a motor that is rated with Full Load Amps (FLA) at 20 amperes may only draw 10 amperes after pipe and ductwork is added to the system. In that case, since the motor cannot be electrically overloaded and operate at more than 20 amperes, the system must be set up from the factory to limit current draw in a non-loaded condition. Also, any addition of ductwork or dust conduit reduces the work efficiency of the system. In the foregoing example, the efficiency is reduced by one-half.
System air friction, i.e., air handling losses, increase with longer duct runs, multiple branches, transitions, elbows, smaller dust inlet ports, flexible hose, loading pressure losses across the filter media, and other restrictions. Because of these impairments to air flow, the fan air flow performance is reduced and the work capacity of the system is decreased. Piping or duct systems used to convey waste matter and air to the dust collector are highly variable in the field. For example, tapered main duct designs are used for systems connecting to multiple processing machines. In such a system a multitude of smaller ducts empty into fewer progressively larger ducts as an attempt to maintain a generally constant conveying velocity throughout the conduit system. The system achieves this, in theory, by keeping an equal cross sectional area of the ductwork for each segment that the air passes through. For smaller portable or stand alone applications, a flexible ducting or flex hose is frequently used for the various tools. The flex hoses are connected, in turn, to many dust ports of varying size for the different tools. Any increase in pipe length or decrease of pipe diameters will result in increased friction loss and reduced airflow. The resulting drop in flow efficiency decreases the amperage draw and this reduces the output power available from the motor.
As mentioned before, in any woodshop environment or in any industrial environment where the process dust involves flammable materials, some measure must be taken for suppressing or arresting flame that may result from inadvertent ignition of the dust in the process air stream.
A flame arrester, deflagration arrester, or flame trap is a piece of equipment installed in an industrial process to stop the propagation of a deflagration traveling along a pipeline by extinguishing the flame. Flame arresters are used on storage tank vents, fuel gas pipelines, storage cabinets, the exhaust system of internal combustion engines, Davy lamps and ovenproof drums.
A flame arrester functions by forcing a flame front through channels that are too narrow to permit the continuance of a flame. The reduced heat of combustion extinguishes the flame from a deflagration, not allowing it to exit the device where it could ignite a secondary deflagration. These passages can be regular, like wire mesh, or irregular, such as those in random packing or tight screening where the heat from the flame propagation is conducted to the metal screen or mesh lowering the heat output and containing the flame front.
The problems that the present invention is directed to concerns deflagrations that may occur in dust collection systems that use filters, (typically pleated cartridge filters) and which may result from dust extracted from dust generating tools and processes. Flammable dust in suspension when ignited can deflagrate or burn quickly producing flame propagation radiating out from the source of combustion. Dust collection systems that experience this type of upset can explode unless adequately protected or vented to atmosphere. The objective here is to eliminate or reduce flame propagation from a deflagration that is vented through a filter (typically a pleated cartridge filter). The embodiments of this invention lend themselves well to cyclone-style collectors (but are not limited to this style) where the cyclone-collected material drops into a collection drum and the separated air flows to a secondary filter and exits to ambient air. The conventional technique employs a semi-passive system where the deflagration has to be diverted through a pressure panel or controlled gate to a flame-quenching or flame-squelching device (i.e., “quelching”). This can be very expensive and complex, requiring gates, dampers, and/or abort gates that have to be timed electronically to actuate within small fractions of a second. Cyclone systems are particularly difficult to adapt to the conventional technique because the flame from a deflagration must be diverted, but at the same time the outlet flow from the system must be simultaneously blocked off. Conventional anti-deflagration systems require complex and expensive sensors and controls, with elaborate engineering control equipment.